Method of manufacturing grain-oriented silicon steel sheet exhibiting excellent magnetic characteristics over the entire length of coil thereof

ABSTRACT

A method of manufacturing a grain-oriented silicon steel sheet exhibiting excellent magnetic characteristics over the entire length of a coil thereof, which involves hot rolling a silicon steel slab containing aluminum and suitable for making a grain-oriented silicon steel sheet; annealing the steel sheet, as the need arises; cold rolling the steel sheet to a final thickness, the cold rolling including an intermediate annealing process; performing a heat effect treatment before, during or after the cold rolling; performing a decarburizing annealing; and performing a final annealing process. The method inhibits oxidation of the surfaces of the steel sheet during the cold rolling.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing agrain-oriented silicon steel sheet exhibiting excellent magneticcharacteristics, and, more particularly, a method of stabilizing themagnetic characteristics in the lengthwise direction of a coil of agrain-oriented silicon steel sheet.

2. Description of the Related Art

Grain-oriented silicon steel sheet is used in transformer cores,generators and the like, and therefore requires excellent, magneticcharacteristics such as high magnetic flux density (usually indicated byvalue B₈ at a magnetic-field intensity of 800 A/m) and small iron loss(usually indicated by 50 Hz alternating iron loss value W_(17/50) at themaximum magnetic flux density of 1.7 T).

Much work has gone into minimizing iron loss in grain-oriented siliconsteel, and improvements have resulted from (1) reducing the thickness ofthe steel sheet, (2) increasing Si content, and (3) reducing thediameters of crystal grains. Such steps have enabled the production of amaterial that exhibits an iron loss W_(17/50) of only 0.90 W/kg.

However, reducing iron loss even further has proven difficult becausefurther reductions in the steel sheet thickness causes defects to ariseduring secondary recrystallization, thus increasing iron loss.Similarly, reducing crystal grain diameters below an average diameter ofabout 4 mm to 8 mm also causes iron-loss-increasing defects to ariseduring secondary recrystallization. Moreover, increasing Si contentnegatively affects the ease with which cold rolling can be performed.

However, by using a so-called magnetic domain refining technique inwhich a local distortion is introduced into the surface of the steelsheet, or grooves are formed on the same, iron loss can be considerablyreduced.

That is, in the case of the foregoing material having an iron lossW_(17/50) of 0.90 W/kg, introduction of appropriate local distortion onthe surface of the steel sheet (by a plasma jetting method or the like)has reduced iron loss of 0.80 W/kg. This magnetic domain refiningtechnique also eliminates the need to reduce crystal grain diameters inthe final product, as is required in conventional techniques. Thequality of material produced through the magnetic domain refiningtechnique depends upon the thickness of the steel sheet, the Si content,and the magnetic flux density.

Since Si content cannot be increased without negatively affecting theworking properties necessary for the steel, minimization of iron lossrequires increasing the magnetic flux density of a thin material.

To improve the magnetic flux density of a grain-oriented silicon steelsheet, the orientation of crystal grains of the product must be highlyintegrated in orientation (110) [001], known as the Goss orientation.Such Goss oriented grains can be obtained through a secondaryrecrystallization phenomenon created during a final annealing process.

In such a secondary recrystallization, selective crystal grain growth ispromoted in crystal grains having the orientation (110) [001], whilegrowth of crystal grains in other orientations is minimized by adding aninhibitor. The inhibitor forms a fine deposited and dispersed phase inthe steel, thereby selectively inhibiting growth of grains.

Since the selective growth of Goss oriented grains produces a materialexhibiting high magnetic flux density, there has been much research anddevelopment regarding inhibitors. A particularly effectively AlNinhibitor has been disclosed in Japanese Patent Publication No.46-23820, wherein a steel sheet containing Al is subjected to a rapidcooling process after it has been annealed but before a final coldrolling process is performed. The final cold rolling is performed usinga high rolling reduction ratio of 80% to 95% to produce a steel sheethaving a thickness of 0.35 mm and a high magnetic flux density B₁₀ of1.981 T (B₈ of about 1.95 T).

However, steel sheet produced by the above-described method suffers fromthe problem that high magnetic flux density cannot be maintained whenthe sheet thickness is reduced.

That is, (110) [001]oriented grains, which form the nuclei of thesecondary recrystallization, are not distributed uniformly in thedirection of the thickness of the steel sheet. Instead, the grains areconcentrated near the surface layer of the steel sheet. Therefore, ifthe thickness of the sheet is reduced, (110) [001]orientated grains arereadily affected by the atmosphere in which the final annealing processis performed, such that the secondary recrystallization becomesunstable. Thus, a method of stabilizing the magnetic characteristics hasbeen widely sought after.

Accordingly, a variety of techniques for manufacturing grain-orientedsilicon steel sheet having excellent and table magnetic characteristicshave been developed. For example, a technique in which an aging heattreatment is performed at 50° C. to 350° C. for one or more minutesduring the rolling process (Japanese Patent Publication No. 54-13846), atechnique in which the steel sheet is maintained at 300° C. to 600° C.for 1 to 30 seconds during the cold rolling process (Japanese PatentPublication No. 54-29182) and a warm rolling technique in which thetemperature of the inlet portion of the rolling stand is controlled to150° C. to 300° C. have all been developed. However, all of theforegoing techniques are unsatisfactory methods for manufacturingindustrial products because, while the coils manufactured from steelsmade in accordance with the above-described techniques exhibit excellentmagnetic characteristics at either end of the coils (the leading andtailing ends of the steel), the magnetic characteristics in the centralportion of the coil are deteriorated.

As described above, if a warm rolling process (for raising thetemperature of the steel sheet) or an aging heat treatment is performedduring the cold rolling of a grain-oriented silicon steel sheetcontaining Al, the magnetic flux density markedly deteriorates exceptthe two ends of the product.

After investigating the foregoing problem, we discovered that, althoughthe secondary recrystallization is completed in all regions of theproduct, the orientation of the crystal grains in the regions in whichthe magnetic flux density deteriorates departs considerably from theorientation (110) [001].

As shown in FIG. 1, the measured change in the angle of deviation inplane from the orientation [001](the "angle of deviation" is hereinafterreferred to as "angle α") increases except the two ends of the coil,thus causing the magnetic flux density to be lowered.

This phenomenon occurs when cold rolling is performed at a warmtemperature range from about 100° C. to 300° C., or when an aging orheat treatment is performed during the rolling process. The foregoingphenomenon often takes place in inverse proportion to the thickness ofthe steel sheet.

OBJECTS OF THE INVENTION

An object of the present invention is to provide a method ofadvantageously manufacturing a grain-oriented silicon steel sheet thatis capable of maintaining excellent magnetic characteristics throughoutthe overall length of a coil of a grain-oriented silicon steel.[.plate.]. .Iadd.sheet .Iaddend.even when a heat effect treatment, suchas a warm rolling process or a heat treatment for aging, is employedduring cold rolling of a grain-oriented silicon steel .[.plate.]..Iadd.sheet .Iaddend.containing Al.

Other objects of the invention will become apparent from the descriptionprovided.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided amethod of manufacturing a grain-oriented silicon steel .[.plate.]..Iadd.sheet .Iaddend.exhibiting excellent magnetic characteristics overthe entire length of a coil thereof. The method involves hot-rolling asilicon steel slab that contains aluminum into a steel .[.plate.]..Iadd.sheet.Iaddend., annealing the steel .[.plate.]. .Iadd.sheet.Iaddend.as the need arises, and cold rolling the steel .[.plate.]..Iadd.sheet .Iaddend.at least once to a final thickness, the coldrolling .[.operation.]. .Iadd.comprising one stage or two stages.Iaddend.including an intermediate annealing process. A heat effecttreatment is also performed before, during or after the cold rollingprocess. A decarburizing annealing process is then performed, followedby a final annealing process. Oxidation of the steel .[.plate.]..Iadd.sheet .Iaddend.surface is thereby inhibited during the coldrolling process.

According to another aspect of the present invention, a method ofmanufacturing a grain-oriented silicon steel .[.plate.]. .Iadd.sheet.Iaddend.exhibiting excellent magnetic characteristics over the entirelength of a coil thereof is provided. The method involves limiting theconcentration of oxygen in the atmosphere in which the heat effecttreatment is performed to about 10 vol % or lower.

According to another aspect of the present invention, a method ofmanufacturing a grain-oriented silicon steel .[.plate.]. .Iadd.sheet.Iaddend.exhibiting excellent magnetic characteristics over the entirelength of a coil thereof is provided. The method involves performing aprocess for inhibiting local oxidation of the steel .[.plate.]..Iadd.sheet .Iaddend.surface occurring when a cold rolling process thatincludes the heat effect treatment is performed.

According to another aspect of the present invention, a method ofmanufacturing a grain-oriented silicon steel .[.plate.]. .Iadd.sheet.Iaddend.exhibiting excellent magnetic characteristics over the entirelength of a coil thereof is provided. The method involves reducing theliquid existing on the surfaces of the steel .[.plate.]. .Iadd.sheet.Iaddend.by a process performed for at least one pass among rollingpasses in the cold rolling process. The process inhibits oxidation beingperformed in a .Iadd.downstream .Iaddend.region from the .[.dischargeside.]. .Iadd.roll bite outlet .Iaddend.of the rolling process to theposition at which the steel .[.plate.]. .Iadd.sheet .Iaddend.is wound.

According to another aspect of the present invention, a method ofmanufacturing a grain-oriented silicon steel .[.plate.]. .Iadd.sheet.Iaddend.exhibiting excellent magnetic characteristics over the entirelength of a coil thereof is provided. The method involves adding aninhibitor for inhibiting oxidation of a steel .[.plate.]. .Iadd.sheet.Iaddend.to rolling oil, roll coolant oil and/or strip coolant oil usedin the cold rolling process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing distribution of magnetic flux densities B₈along the lengthwise direction of a coil produced in accordance with aprior art method, and the distribution of deviation angles α from theorientation (110) [001]along the lengthwise direction of a coil;

FIG. 2 is a graph showing the relationship between the quantity ofnitriding of the steel .[.plate.]. .Iadd.sheet .Iaddend.measuredimmediately before secondary recrystallization is initiated and themagnetic flux density measured after the secondary recrystallization hasbeen performed;

FIG. 3 is a graph showing influence of the concentration of O₂ in theatmosphere for the aging heat treatment upon the quantity of nitridingin the steel immediately before the secondary recrystallization, thedeviation angle α of the secondarily recrystallized grains subjected tothe final annealing process, and magnetic characteristics (B₈ andW_(17/50)) of the product steel; and

FIG. 4 is a graph showing influence of 0 to 4 cold rolling passes inwhich a liquid removal process according to the invention have beenperformed upon the magnetic characteristics (B₈ and W_(17/50)) of theproduct steel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In our investigations, we discovered that during the final annealingprocess, a change in the nitrogen components along the lengthwisedirection of the coil occurs. That is, after performing the finalannealing process, the content of nitrogen at the two ends of the coilremained substantially unchanged, but an increase in nitrogen content of3 ppm to 15 ppm in the other portions was observed.

In the case of grain-oriented silicon steel .[.plate.]. .Iadd.sheet.Iaddend.containing Al, the initial stage of the final annealing processis performed in an atmosphere containing nitrogen to "nitride" the steel.[.plate.]. .Iadd.sheet.Iaddend.. However, what influence nitriding hadon the secondary recrystallization had been unclear.

Therefore, we investigated the influence of nitriding upon secondaryrecrystallization, and in particular its effect on the magnetic fluxdensity of the product steel.

FIG. 2 shows results of investigation of the relationship between themagnetic flux density observed after secondary recrystallization andincreases in the quantity of nitrogen (the quantity of nitriding)created by the nitriding process. To conduct the investigation, a.[.grain-oriented.]. silicon steel .[.plate.]. .Iadd.sheet.Iaddend.containing Mn by 0.07 wt %, Al by 0.025 wt %, Sb by 0.025 wt %,Se by 0.020 wt % and N by 0.0085 wt %, which had been decarburized,primary-recrystallized and annealed, was subjected to a nitridingprocess at 750° C. for 30 seconds in an atmosphere in which NH₃ was, ata variety of ratios, mixed with gas consisting of 50 vol % N₂ and 50 vol% H₂ .[./.]..Iadd.. .Iaddend.Test samples in which the content ofnitrogen in the steel was thusly raised were thensecondary-recrystallized in an experiment chamber.

As can be seen in FIG. 2, increases in the quantity of nitriding in thesteel caused decreases in magnetic flux density. Notably, if thequantity of nitrogen exceeded 10 ppm, the magnetic flux density of thesteel was sharply reduced.

The investigation confirmed that deterioration in the magnetic fluxdensity was caused by nitriding of the steel .[.plate.]. .Iadd.sheet.Iaddend.at the time of the final annealing process. Furthermore, aclose relationship between magnetic flux deterioration observed in thesteel .[.plate.]. .Iadd.sheet .Iaddend.and the method of cold rollingwas found.

In another investigation, five hot-rolled coils, each of which was madeof .[.grain-oriented.]. silicon steel that contained C by 0.075 wt %, Siby 3.26 wt %, Mn by 0.07 wt %, P by 0.006 wt %, Al by 0.027 wt %, Sb by0.025 wt %, Se by 0.020 wt % and N by 0.0085 wt %, were annealed at1000° C. for 90 seconds; the hot-rolled coils were then .[.washed.]..Iadd.pickled .Iaddend.with an acid; cold rolled (as a first coldrolling process) to have a thickness of 1.50 mm; subjected to anintermediate annealing process at 1120° C. for 60 seconds; rapidlycooled with .[.mist.]. .Iadd.water.Iaddend.; again .[.washed.]..Iadd.pickled .Iaddend.with an acid; and cold rolled a second time tohave a thickness of 0.22 mm.

When the thickness of the steel .[.plate.]. .Iadd.sheet .Iaddend.was at0.75 mm during the second cold rolling process, an aging heat treatmentwas performed at 300° C. for 2 minutes. At this time, the followingatmospheres were employed for the aging heat treatment, each atmospherefor a different coil:

(1) gas consisting of N₂ by 100 vol %

(2) gas consisting of N₂ by 95 vol %+O₂ by 5 vol %

(3) gas consisting of N₂ by 91 vol %+O₂ by 9 vol %

(4) gas consisting of N₂ by 87 vol %+O₂ by 13 vol %

(5) gas consisting of N₂ by 79 vol %+O₂ by 21 vol %

The oxygen and nitrogen content in each steel .[.plate.]. .Iadd.sheet.Iaddend.subjected to the cold rolling process were determined asfollows:

(1) O: 28 ppm, N: 86 ppm

(2) O: 26 ppm, N: 86 ppm

(3) O: 27 ppm, N: 85 ppm

(4) O: 25 ppm, N: 86 ppm

(5) O: 27 ppm, N: 85 ppm

None of the steel .[.plates.]. .Iadd.sheets .Iaddend.exhibited anincrease in nitrogen content (no nitriding took place), and no residualscale was observed.

Then, the steel .[.plates.]. .Iadd.sheets .Iaddend.weredecarburizing-annealed at 850° C. for 2 minutes in a continuousannealing furnace, the atmosphere consisting of 55 vol % H₂, the balancesubstantially consisting of N₂. The dew-point was 48° C. The weight ofoxygen per unit area of the individual .[.plates.]. .Iadd.sheets.Iaddend.was then measured, with the following results: (1) 1.18 g/m²,(2) 1.22 g/m², (3) 1.25 g/m², (4) 1.48 g/m², and (5) 1.75 g/m². Thus, itwas confirmed that oxidation of the steel .[.plates.]. .Iadd.sheets.Iaddend.proceeded in proportion to the concentration of oxygen in theatmosphere in which the aging heat treatment was performed.

After the decarburizing annealing process had been performed, anannealing .[.separation agent.]. .Iadd.separator.Iaddend., consisting ofTiO₂ and Sr(OH)₂ •8H₂ O added to MgO by 5 wt % and 3 wt % respectively,was applied to the surface of each of the steel .[.plates.]..Iadd.sheets.Iaddend.; each of the steel .[.plates.]. .Iadd.sheets.Iaddend.was then divided into two sections in the lengthwise direction;and each of the sections was wound into the form of a coil. Thetemperature of .Iadd.the .Iaddend.first of the divided coils in eachpair was, in an atmosphere of N₂, maintained at 830° C. for 40 hours,then raised to 1200° C. at a rate of 12° C./hour in an atmosphereconsisting of 25 vol % N₂ and 75 vol % H₂ ; and then final annealing wasperformed such that the temperature was maintained at 1200° C. for 10hours in an atmosphere of H₂, after which the temperature was lowered.The second coil in each pair was maintained at a temperature of 830° C.for 40 hours in an atmosphere of N₂ ; the temperature was raised to 950°C. (just below the temperature where secondary recrystallization isinitiated) at a temperature rising rate of 12° C./hour in an atmosphereof 25 vol % N₂ and 75 vol % H₂, after which the temperature wasimmediately lowered.

The first coil of each pair, having been subjected to the finalannealing process, was also subjected to a process which removednon-reacted portions of the .[.separation agent.]..Iadd.separator.Iaddend.. Then, a sample was taken from the centralportion of the first coil in the lengthwise direction of the same tomeasure the magnetic characteristics and the crystallization orientationangle α. The second coil of each pair, which did not undergo secondaryrecrystallization, was also subjected to the process which removednon-reacted portions of the .[.separation agent.]..Iadd.separator.Iaddend.. A sample was then taken from the centralportion of the coil in the lengthwise direction of the same; and thecontent of nitrogen was measured.

Results with respect to the concentration of O₂ in the atmosphere forthe aging heat treatment are collectively shown in FIG. 3.

As revealed in FIG. 3, if the content of oxygen in the atmosphere forthe aging heat treatment is lower than 10 vol %, the deterioration inthe magnetic characteristics occurring in the central portion of coilsproduced by conventional techniques can effectively be prevented.

Why an increase in the concentration of oxygen in the atmosphere for theaging heat treatment promotes nitriding of the steel .[.plate.]..Iadd.sheet .Iaddend.during the final annealing process will now bedescribed.

When conventional heat effect treatments are performed before, during orafter the rolling process, water and oxygen in liquids on the surface ofthe steel .[.plate.]. .Iadd.sheet .Iaddend.(such as rolling oil orcoolant oil) cause local oxidation to take place on the surface of thesteel .[.plate.]. .Iadd.sheet.Iaddend.. The local oxidation isexacerbated when the temperature of the steel .[.plate.]. .Iadd.sheet.Iaddend.is raised.

The local oxidation results in non-uniform concentration of elements atthe extreme upper surface of the steel .[.plate.]. .Iadd.sheet.Iaddend..

Consequently, non-uniform dispersion of oxide particles results insub-scales being formed in the surface layers of the steel .[.plate.]..Iadd.sheet .Iaddend.in the subsequent decarburizing annealing process,whereby nitriding of the steel .[.plate.]. .Iadd.sheet .Iaddend.proceedslocally during the final annealing process in the portions havingrelatively low concentrations of oxide particles.

Moreover, non-uniform dispersion of oxide particles takes place in thesub-scales formed on the surface layers of the steel .[.plate.]..Iadd.sheet .Iaddend.in any subsequent decarburizing annealing process,causing areas having relatively low concentrations of oxide particles tobe generated locally, thereby allowing oxygen and nitrogen atoms to beeasily diffused.

As a result, nitriding occurs in the final annealing process, thusresulting in deterioration of the steel .[.plate's.]. .Iadd.sheet's.Iaddend.magnetic characteristics.

In such a steel .[.plate.]. .Iadd.sheet.Iaddend., low concentrations ofoxide particles allows oxygen atoms to easily diffuse in the steelduring the decarburizing annealing process. Thus, oxidation is promotedand the weight of oxygen per unit area of the surface of the steel.[.plate.]. .Iadd.sheet .Iaddend.increases.

The foregoing discoveries have provided the basis for the presentinvention.

In the present invention, there are three types of heat effecttreatments contemplated: one which is performed before the cold rollingprocess, another which is performed during the cold rolling process, anda third which is performed after the cold rolling process.

The heat effect treatment performed before the cold rolling processrefers to a coil heating process performed before the coil is.[.cooled.]. .Iadd.cold rolled.Iaddend.. This heat effect treatment isemployed when the cold rolling process is performed in a warm condition.

The heat effect treatment performed during the cold rolling processrefers particularly to either a "warm rolling" process for maintainingthe steel temperature during the cold rolling process, an aging heattreatment performed between cold rolling passes, or a process formaintaining the temperature when the coil is wound between cold rollingpasses.

The heat effect treatment to be performed after the cold rolling processrefers to a process for maintaining the temperature at which the coil iswound after cold rolling has been performed.

The composition ranges for components of a steel slab to which thepresent invention can appropriately be applied will now be described.

C: about 0.01 wt % to 0.10 wt %

Carbon improves the hot-rolled structure such that secondaryrecrystallization is promoted. Therefore, the steel must contain atleast about 0.01 wt % of carbon. If the steel contains more than about0.10 wt % of carbon, the carbon cannot easily be removed bydecarburizing annealing, thereby deteriorating the magneticcharacteristics of the product steel. As a result, it is preferable thatthe carbon content be in a range from about 0.01 wt % to 0.10 wt %.

Si: about 2.0 wt % to 6.5 wt %

Silicon .[.strengthens.]. .Iadd.increases .Iaddend.the electricresistance of the steel, which .[.prevents.]. .Iadd.lowers .Iaddend.ironloss. Therefore, the steel must contain about 2.0 wt % or more silicon.If the silicon content is larger than about 6.5 wt %, the rollingprocess cannot easily be performed. Thus, it is preferable that the Sicontent be in a range from about 2.0 wt % to 6.5 wt %.

Mn: about 0.04 wt % to 2.0 wt %

Manganese prevents brittleness in the steel .[.plate.]. .Iadd.sheet.Iaddend.when the hot rolling process is performed. To achieve thiseffect, the Mn content must be about 0.04 wt % or more. If the Mncontent is larger than about 2.0 wt %, the decarburizing process cannotbe performed smoothly. Therefore, it is preferable that Mn content be ina range from about 0.04 wt % to 2.0 wt %.

Al: about 0.01 wt % to 0.04 wt %

Aluminum, as a component of AlN, serves as an inhibitor to inhibit thegrowth of normal grains. If the Al content is less than about 0.01 wt %,the desired inhibition effect is not obtained. If the Al content islarger than about 0.04 wt %, .[.deposited.]..Iadd.precipitated.Iaddend.AlN is coarsely enlarged, thereby deteriorating the inhibitioneffect. Therefore, it is preferable that the Al content be in a rangefrom about 0.01 wt % to 0.04 wt %.

N: about 0.003 wt % to 0.010 wt %

Nitrogen, like aluminum, is a component of AlN, and therefore must becontained in the steel in an amount of about 0.003 wt % or more. If theN content is larger than about 0.010 wt %, .[.deposited.]..Iadd.precipitated .Iaddend.AlN is coarsely enlarged and the inhibitioneffect deteriorates. Therefore, it is preferable that the N content bein a range from about 0.003 wt % to 0.010 wt %.

To enhance the inhibition effect, components S, Se, Sb, B, Sn, Cu, Bi,Te, Cr and Ni may also be added. To improve the inhibition effect, it ispreferable that each of S, Se, Sb, Bi and Te be added in a range ofabout 0.005 wt % to 0.050 wt %, each of Sn, Cu, Cr and Ni be added in arange of about 0.03 wt % to 0.30 wt %, and B be added in a range ofabout 0.0003 wt % to 0.0020 wt %.

A manufacturing process illustrating the present invention will now bedescribed. The description is not intended to limit the inventiondefined in the appended claims.

A steel slab having the above-described preferred composition range issubjected to a heating process to prepare the slab for hot rolling andfor forming the inhibitor into a solid solution. Then, the steel slab ishot-rolled so that a hot-rolled coil is manufactured. The hot-rolledcoil is, as the need arises, subjected to .[.a hot rolling.]. .Iadd.an.Iaddend.annealing process, and then is cold rolled one .[.or twotimes.]. .Iadd.stage or two stages .Iaddend.to a final thickness, thecold rolling including an intermediate annealing process. To improve themagnetic characteristics of the steel .[.plate.]. .Iadd.sheet.Iaddend.,a warm rolling and an aging heat treatment are performed at this time.

The aging heat treatment performed between rolling passes includes aheat treatment of short duration using a continuous furnace; the agingis accomplished by using the sensible heat of the coil when the coil iswound after the rolling process has been performed. Another heattreatment is performed on the coil for an extended time in a BOXfurnace. The concentration of oxygen in the atmosphere during the heattreatment is limited to about 10 % or lower.

A process for inhibiting local oxidation on the surface of the steel.[.plate.]. .Iadd.sheet .Iaddend.according to the present invention isalso performed. As a result, a grain-oriented silicon steel .[.plate.]..Iadd.sheet .Iaddend.is produced that exhibits excellent magneticcharacteristics over the entire length of a coil thereof.

According to the present invention, there may be employed any of thefollowing warm rolling methods: heating the coil before the coil isrolled; limiting the use of rolling oil used in lubricating the rollsand for cooling the coil such that heat generated during the rollingoperation is used in a warm rolling process; and a method combining theforegoing two methods. The rolling machine may be a .[.reverse-typemachine.]. .Iadd.reverse mill.Iaddend., such as a Sendzimer mill, or a.[.continuous-type machine.]. .Iadd.continuous mill.Iaddend., such as a.[.tandem-type.]. .Iadd.tandem .Iaddend.mill.

According to the present invention, the concentration of oxygen islimited to about 10 vol % or lower in any of the atmospheres in whichthe coil is heated before the coil is rolled, in which the coil is woundand retained between rolling passes, or in which the coil is wound andretained after the coil has been rolled. As a result, a grain-orientedsilicon steel .[.plate.]. .Iadd.sheet .Iaddend.can be obtained thatexhibits excellent magnetic characteristics over the entire length of acoil thereof.

If the concentration of oxygen in the atmosphere used in the heat effecttreatment is higher than about 10 vol %, the surface of the rolled steel.[.plate.]. .Iadd.sheet .Iaddend.is easily oxidized and nitrided.Consequently, nitriding proceeds during the final annealing process,thereby deteriorating the magnetic characteristics of the coil except ateither end of the coil. Thus, it is important to limit the concentrationof oxygen in the heat effect treatment atmosphere to about 10 vol % orlower.

As for components other than oxygen in heat effect treatmentatmospheres, it is preferable that a neutral atmosphere of N₂ or Ar beemployed. However, a reducing atmosphere comprising a mixture of H₂, COor CO₂ is also permitted.

After cold rolling, the coil of the present invention is subjected to aconventional decarburizing annealing process, followed by theapplication of an annealing .[.separation agent.]..Iadd.separator.Iaddend.. The coil is then subjected to the finalannealing process, including the secondary recrystallization andannealing for purification. After the final annealing process has beencompleted, non-reacted portions of the .[.separation agent.]..Iadd.separator .Iaddend.are removed, followed by an application ofan.[...]. insulating coating, as the need arises. Finally, the steel issubjected to a flattening heat treatment.

A means according to the present invention for inhibiting the localoxidation of the surface of the steel .[.plate.]. .Iadd.sheet.Iaddend.involves performing at least one oxidation inhibiting processpass as part of the rolling passes for the cold rolling process. Theoxidation inhibiting process pass reduces the liquid existing on thesurface of the steel .[.plate.]. .Iadd.sheet .Iaddend.and is performedin a .Iadd.downstream .Iaddend.region ranging from the .Iadd.roll bite.Iaddend.outlet of the rolling process to the position at which thesteel .[.plate.]. .Iadd.sheet .Iaddend.is wound.

As a result of the foregoing oxidation inhibiting process, the quantityof the water screen existing on the surface of the steel .[.plate.]..Iadd.sheet .Iaddend.is reduced, as well as the total quantity ofdissolved oxygen existing in water. Therefore, local oxidation of thesteel .[.plate.]. .Iadd.sheet .Iaddend.is effectively inhibited. As amatter of course, it is preferable that the foregoing oxidationinhibiting process be performed in every rolling pass.

Another means for inhibiting the local oxidation of the steel.[.plate.]. .Iadd.sheet .Iaddend.is to cause an oxidation inhibitingagent to be contained in liquid existing on the surface of the steel.[.plate.]. .Iadd.sheet.Iaddend..

This can be accomplished by adding the oxidation inhibiting agent to therolling oil, the roll coolant oil and/or the strip coolant oil used inthe cold rolling process.

Examples of oxidation inhibiting agents include aliphatic amine oftallow, sorbitan mono-oleate, ester of succinic acid and the like. Otherinhibiting agents may also be employed.

Although any of the above-described means for inhibiting local oxidationon the surface of the steel .[.plate.]. .Iadd.sheet .Iaddend.provides asatisfactory effect, employment of two or more means can enhance theeffect obtained.

After the steel .[.plate.]. .Iadd.sheet .Iaddend.has been rolled to afinal thickness by the above-described cold rolling process, aconventional decarburizing annealing process is performed, followed bythe application of an annealing .[.separation agent.]. .Iadd.separator.Iaddend.to the steel .[.plate.]. .Iadd.sheet.Iaddend.. Then, the steelsheet is subjected to a final annealing process in which secondaryrecrystallization and annealing for purification are performed.

After the final annealing process has been performed, non-reactedportions of the separator are removed, and thereafter an insulatingcoating material is applied. Finally, a flattening heat treatment isperformed, if needed.

It is also understood that a magnetic domain refining process, such asirradiation with laser beams or plasma irradiation, may be performed.

Other and further objectives, features and advantages of the inventionwill become apparent from the following description.

EXAMPLES

The invention will now be described through illustrative examples. Theexamples are not intended to limit the invention defined in the appendedclaims.

EXAMPLE 1

A steel slab, containing C by 0.07 wt %, Si by 3.25 wt %, Mn by 0.07 wt%, S by 0.004 wt %, Al by 0.028 wt %, Sb by 0.028 wt % and N by 0.007 wt% and the balance substantially consisting of Fe, was heated to 1250° C,then hot rolled to produce a hot-rolled steel sheet having a thicknessof 1.8 mm. Then, the steel sheet was subjected to an annealing at 1150°C. for one minute, followed by a pickling. The steel sheet was dividedinto two coils, and each coil was cold rolled with six passes by aSendzimir mill so that it had a final thickness of 0.20 mm. At thistime, the first coil was subjected to a warm rolling process in whichthe quantity of the rolling oil was limited so as to raise thetemperature of the rolled steel sheet after the second pass from 150° C.to 220° C.

The second coil was subjected to a process which maintained thetemperature at which the coil was wound after the cold rolling processhad been performed. This process involved surrounding the windingapparatus with a box-type structure into which N₂ gas was injected sothat the concentration of oxygen in the atmosphere was limited tobetween 1 vol % to 5 vol %.

The second coil was wound according to a conventional technique inambient atmosphere.

Then, both of the coils were degreased and subjected to thedecarburizing annealing process at 850° C. for 2 minutes in anatmosphere of 40 vol % H₂, the dew point of the atmosphere being 50° C.Then, MgO containing TiO₂ by 5 wt % and Sr(OH)₂ •8H₂ O by 3 wt % was, asan annealing separator, applied to the coils, after which the coils werewound into coil form. Then, the coils were subjected to the finalannealing process.

The final annealing process was performed such that the temperature ofthe coils were maintained at 850° C. for 15 hours in an atmosphere ofN₂, after which the temperature was raised to 1200° C. at a rate of 15°C./hour in an atmosphere of 25 vol % N₂ and 75 vol % H₂. Then, thetemperature was maintained at 1200° C. for 5 hours in an atmosphere ofH₂.

After the final annealing process had been performed, non-reactedportions of the separator were removed from each of the coils, and thentension coating liquid containing magnesium phosphate and colloidalsilica was applied. Thereafter, a flattening annealing process whichalso baked the coating material was performed at 800° C. for 1 minute.

Results of the magnetic characteristic evaluations of the leadingportion, the central portion and the tailing end of each coil are shownin Table 1.

                  TABLE 1                                                         ______________________________________                                                   EXAMPLE OF       COMPARATIVE                                                  THIS INVENTION   EXAMPLE                                                      CONCENTRATION OF OXYGEN                                                       WHEN COIL IS WOUND                                                            1-5 vol %        21 vol %                                          MAGNETIC CHARACTERISTICS                                                      POSITION IN  Bg      W.sub.17/50                                                                              Bg    W.sub.17/50                             THE COIL     (T)     (W/kg)     (T)   (W/kg)                                  ______________________________________                                        LEADING END  1.932   0.783      1.924 0.824                                   CENTRAL      1.935   0.764      1.846 1.093                                   PORTION                                                                       TAILING END  1.933   0.775      1.928 0.816                                   ______________________________________                                    

As shown in Table 1, the conventional coil exhibited deterioration inthe magnetic characteristics in the central portion thereof, whereas nosuch deterioration occurred in the coil according to the presentinvention.

EXAMPLE 2

A steel slab, containing C by 0.078 wt %, Si by 3.35 wt %, Mn by 0.07 wt%, S by 0.007 wt %, Al by 0.028 wt %, Se by 0.020 wt %, Sb by 0.025 wt %and N by 0.007 wt %, with the balance substantially consisting of Fe,was heated to 1420° C., then hot rolled to form a hot-rolled steel sheethaving a thickness of 2.2 mm. Then, the steel sheet was subjected to anannealing process at 1000° C. for 50 seconds, followed by a pickling anda first cold-rolling process to achieve an intermediate thickness of 1.5mm. Then, the coil was subjected to intermediate annealing at 1150° C.for one minute, followed by a pickling. The coil was then divided intotwo sections.

The formed coils were subjected to a second cold rolling process so thateach of the coils had a final thickness of 0.22 mm. At the point in thesecond cold-rolling process where the coil thickness was 0.75 mm, thecoils were subjected to an aging heat treatment at 200° C. for one hour.The heat treatment for aging was performed such that the concentrationof oxygen in the atmosphere in the heating BOX furnace for one coil waslowered to between 0.01 wt % and 0.5 wt % by injecting Ar. Conversely,the other coil was inserted into a BOX furnace having an ambientatmosphere, as is done in conventional techniques.

Thereafter, both of the coils were degreased and subjected todecarburizing annealing at 850° C. for 2 minutes in an atmosphere of 60vol % H₂ with the balance substantially consisting of N₂, the dew pointof the atmosphere being 55° C. Then, MgO containing TiO₂ by 8 wt % andSrSO₄ by 3 wt % was, as an annealing separator, applied to the coils,and thereafter the coils were wound into coil form. Then, the formedcoils were subjected to a final annealing process.

The final annealing process was performed such that the temperature ofeach coil was maintained at 840° C. for 40 hours in an atmosphere of N₂,and then the temperature was raised to 1200° C. at a rate of 15° C./hourin an atmosphere consisting of 25 vol % N₂ and 75 vol % of H₂. Then, thetemperature was maintained at 1200° C. for 5 hours in an atmosphere ofH₂.

After the final annealing process had been completed, non-reactedportions of the separator were removed from the two coils, and tensioncoating liquid containing magnesium phosphate and colloidal silica wasapplied. Then, a flattening annealing process, which also baked thecoated material, was performed at 800° C. for one minute.

Results of the magnetic characteristic evaluations of the leadingportion, the central portion and the tailing end of each coil are shownin Table 2.

                  TABLE 2                                                         ______________________________________                                                   EXAMPLE OF       COMPARATIVE                                                  THIS INVENTION   EXAMPLE                                                      CONCENTRATION OF OXYGEN                                                       IN THE ATMOSPHERE FOR                                                         HEAT TREATMENT FOR AGING                                                      0.01-0.5 vol %   21 vol %                                          MAGNETIC CHARACTERISTICS                                                      POSITION IN  Bg      W.sub.17/50                                                                              Bg    W.sub.17/50                             THE COIL     (T)     (W/kg)     (T)   (W/kg)                                  ______________________________________                                        LEADING END  1.938   0.803      1.932 0.825                                   CENTRAL      1.942   0.795      1.840 1.124                                   PORTION                                                                       TAILING END  1.940   0.801      1.919 0.843                                   ______________________________________                                    

As is shown in Table 2, the conventional coil exhibited deterioration inthe magnetic characteristics in the central portion thereof, whereas nosuch deterioration occurred in the coil according to the presentinvention.

EXAMPLE 3

A steel slab, containing C by 0.075 wt %, Si by 3.26 wt %, Mn by 0.08 wt%, S by 0.016 wt %, Al by 0.022 wt %, and N by 0.008 wt %, with thebalance substantially consisting of Fe, was heated to 1380° C., followedby a hot rolling to produce a hot-rolled steel sheet having a thicknessof 2.2 mm. Then, the steel sheet was subjected to an annealing processat 1150° C. for 50 seconds, followed by a pickling. The coil was thendivided into two sections, and the two coils were rolled by tandemrolling mill to a final thickness of 0.35 mm. Prior to the tandemrolling, the two coils were heated to 250° C., and the quantity of thecoolant was adjusted so as to raise the temperature of the steel sheetduring the tandem rolling from 150° C. to 200° C.

One of the coils was subjected to a heat effect treatment wherein thecoil was heated before tandem rolling. At this time, N₂ was injectedinto the BOX furnace so that the concentration of oxygen ranged between0.05 vol % and 0.6 vol %. The other coil was also subjected to a heateffect treatment wherein the coil was heated before tandem rolling, butthe heating was performed in a BOX furnace having an ambient atmospherein accordance with conventional techniques.

Then, both of the coils were degreased and subjected to decarburizingannealing at 840° C. for 2 minutes in a atmosphere of 50 vol % H₂ withthe balance substantially consisting of N₂, the dew point of theatmosphere being 50° C.; Then, MgO containing TiO₂ by 10 wt % andSr(OH)₂ •8H₂ O by 3 wt % was, as an annealing separator, applied to thecoils, followed by winding the coils into coil form. Then, the formedcoils were subjected to a final annealing process.

The final annealing process was performed such that the temperature wasraised to 850° C. at a rate of 20° C./hour in an atmosphere of N₂. Then,the temperature was raised to 1200° C. at a rate of 15° C./hour in anatmosphere consisting of 25 vol % N₂ and 75 vol % H₂, followed bymaintaining the coils at 1200° C. for 5 hours in an atmosphere of H₂.

After the final annealing process had been completed, non-reactedportions of the separator were removed from the two coils, and tensioncoating liquid containing aluminum phosphate and colloidal silica wasapplied. Then, a flattening annealing process, which also baked thecoated material, was performed at 800° C. for one hour.

Results of the magnetic characteristic evaluations of the leadingportion, the central portion and the tailing end of each coil are shownin Table 3.

                  TABLE 3                                                         ______________________________________                                                   EXAMPLE OF       COMPARATIVE                                                  THIS INVENTION   EXAMPLE                                                      CONCENTRATION OF OXYGEN                                                       IN THE ATMOSPHERE FOR                                                         HEAT TREATMENT FOR AGING                                                      0.05-0.6 vol %   21 vol %                                          MAGNETIC CHARACTERISTICS                                                      POSITION IN  Bg      W.sub.17/50                                                                              Bg    W.sub.17/50                             THE COIL     (T)     (W/kg)     (T)   (W/kg)                                  ______________________________________                                        LEADING END  1.935   1.123      1.930 1.153                                   CENTRAL      1.938   1.105      1.893 1.287                                   PORTION                                                                       TAILING END  1.933   1.117      1.931 1.146                                   ______________________________________                                    

As shown in Table 3, the conventional coil exhibited magneticcharacteristic deterioration in the central portion thereof, whereas nosuch deterioration occurred in the coil produced according to thepresent invention.

EXAMPLE 4

Steel slabs having the variety of compositions shown in Table 4 wereheated to 1410° C., and then hot rolled to produce a hot-rolled steelsheet having a thickness of 2.0 mm. Then, the steel sheet was pickled,and the surface scales were removed, followed by a first cold rolling toproduce a steel sheet having an intermediate thickness of 1.50 mm. Then,the steel sheet was subjected to an intermediate annealing process at1100° C. for 50 seconds, and then water mist was used to rapidly coolthe steel sheet to 350° C. at a cooling rate of 40° C./second. Thetemperature of the steel sheet was maintained at 350° C. for 20 seconds,the temperature thereafter being lowered with water. Then, the surfaceof the steel sheet was ground so that a portion of the surface scaleswas removed, with the sheet then being cold rolled by a Sendzimir millwith six passes to produce a final thickness of 0.22 mm.

At this time, a warm rolling process was performed in which the quantityof rolling oil was limited so as to raise the temperature of the steelsheet from 150° C. to 180° C. after the second pass.

After the cold rolling had been performed, the steel was subjected to aprocess for maintaining the temperature at which the coil was wound. Toachieve this, the apparatus for winding the coil was surrounded by abox-type structure, and Ar gas was injected so as to limit theconcentration of oxygen in the atmosphere to between 1% and 3 %.

Then, the coil was degreased and subjected to decarburizing annealing at850° C. for 2 minutes in a atmosphere of 60 vol % H₂ with the balancesubstantially consisting of N₂, the dew point of the atmosphere being45° C. Then, MgO containing TiO₂ by 5 wt % and Sr(OH)₂ •8H₂ O by 3 wt %was, as an annealing separator, applied to the coil. The coil was thenwound into coil form and subjected to a final annealing process.

The final annealing process involved maintaining the temperature at 850°C. for 20 hours, and then raising the temperature to 1200° C. at a rateof 15° C./hour in an atmosphere consisting of 25 vol % N₂ and 75 vol %H₂, followed by maintaining the coil at 1200° C. for 5 hours in anatmosphere of H₂.

After the final annealing process had been completed, non-reactedportions of the separator were removed from the coil, and tensioncoating liquid containing magnesium phosphate and colloidal silica wasapplied. Then, a flattening annealing process, which also baked thecoated material, was performed at 800° C. for one hour.

Results of the magnetic characteristic evaluations at the leadingportion, the central portion and the tailing end of each coil are shownin Table 5.

                                      TABLE 4                                     __________________________________________________________________________    Unit of *: ppm                                                                STEEL                                                                             COMPOSITION OF ELEMENTS (wt %)                                            No. C  Si Mn P  S  Al Se Cu Ni Cr Sn Mo Sb Bi Te B*                                                                              N*                         __________________________________________________________________________    A   0.07                                                                             3.34                                                                             0.075                                                                            0.008                                                                            0.016                                                                            0.026                                                                            tr 0.01                                                                             0.01                                                                             0.01                                                                             0.02                                                                             tr tr tr tr 3 82                         B   0.08                                                                             3.30                                                                             0.073                                                                            0.005                                                                            0.003                                                                            0.025                                                                            0.018                                                                            0.01                                                                             0.01                                                                             0.01                                                                             0.01                                                                             tr tr tr tr 3 78                         C   0.06                                                                             3.36                                                                             0.072                                                                            0.007                                                                            0.015                                                                            0.025                                                                            tr 0.01                                                                             0.02                                                                             0.01                                                                             0.01                                                                             tr 0.025                                                                            tr tr 3 69                         D   0.07                                                                             3.38                                                                             0.075                                                                            0.012                                                                            0.004                                                                            0.027                                                                            0.020                                                                            0.02                                                                             0.01                                                                             0.02                                                                             0.01                                                                             tr 0.022                                                                            tr tr 4 73                         E   0.07                                                                             3.30                                                                             0.073                                                                            0.006                                                                            0.003                                                                            0.026                                                                            0.020                                                                            0.01                                                                             0.05                                                                             0.01                                                                             0.02                                                                             tr 0.025                                                                            tr tr 2 78                         F   0.08                                                                             3.32                                                                             0.074                                                                            0.008                                                                            0.016                                                                            0.027                                                                            tr 0.08                                                                             0.01                                                                             0.01                                                                             0.02                                                                             tr 0.025                                                                            tr tr 3 79                         G   0.06                                                                             3.30                                                                             0.076                                                                            0.015                                                                            0.018                                                                            0.026                                                                            tr 0.02                                                                             0.08                                                                             0.01                                                                             0.15                                                                             tr 0.020                                                                            tr tr 3 85                         H   0.07                                                                             3.35                                                                             0.068                                                                            0.008                                                                            0.018                                                                            0.026                                                                            tr 0.01                                                                             0.02                                                                             0.01                                                                             0.02                                                                             tr 0.018                                                                            0.008                                                                            tr 4 83                         I   0.08                                                                             3.30                                                                             0.080                                                                            0.003                                                                            0.004                                                                            0.027                                                                            0.016                                                                            0.02                                                                             0.01                                                                             0.01                                                                             0.01                                                                             0.12                                                                             0.025                                                                            0.005                                                                            tr 4 88                         J   0.08                                                                             3.38                                                                             0.088                                                                            0.009                                                                            0.005                                                                            0.024                                                                            0.018                                                                            0.01                                                                             0.01                                                                             0.02                                                                             0.02                                                                             0.08                                                                             0.022                                                                            tr 0.005                                                                            4 82                         K   0.07                                                                             3.39                                                                             0.075                                                                            0.012                                                                            0.003                                                                            0.023                                                                            0.020                                                                            0.01                                                                             0.01                                                                             0.01                                                                             0.02                                                                             tr tr tr tr 12                                                                              65                         L   0.08                                                                             3.35                                                                             0.073                                                                            0.008                                                                            0.004                                                                            0.026                                                                            0.021                                                                            0.01                                                                             0.02                                                                             0.10                                                                             0.02                                                                             tr tr 0.012                                                                            tr 3 75                         M   0.07                                                                             3.30                                                                             0.075                                                                            0.058                                                                            0.004                                                                            0.025                                                                            0.021                                                                            0.01                                                                             0.02                                                                             0.01                                                                             0.10                                                                             tr tr tr tr 4 79                         N   0.07                                                                             3.34                                                                             0.077                                                                            0.058                                                                            0.004                                                                            0.025                                                                            0.020                                                                            0.01                                                                             0.01                                                                             0.02                                                                             0.01                                                                             tr 0.025                                                                            0.015                                                                            tr 4 83                         O   0.07                                                                             3.36                                                                             0.069                                                                            0.008                                                                            0.004                                                                            0.027                                                                            0.021                                                                            0.07                                                                             0.01                                                                             0.02                                                                             0.02                                                                             tr 0.025                                                                            tr tr 3 80                         P   0.07                                                                             3.38                                                                             0.076                                                                            0.005                                                                            0.001                                                                            0.028                                                                            0.018                                                                            0.08                                                                             0.01                                                                             0.02                                                                             0.01                                                                             tr 0.025                                                                            tr tr 4 75                         __________________________________________________________________________

                  TABLE 5                                                         ______________________________________                                        POSITION IN THE COIL                                                          LEADING END     CENTRAL PORTION                                                                             TAILING END                                             Bg      W.sub.17/50                                                                           Bg     W.sub.17/50                                                                          Bg   W.sub.17/50                        STEEL No.                                                                             (T)     (W/kg)  (T)    (W/kg) (T)  (W/kg)                             ______________________________________                                        A       1.926   0.846   1.923  0.848  1.925                                                                              0.849                              B       1.924   0.839   1.922  0.845  1.924                                                                              0.846                              C       1.938   0.790   1.936  0.810  1.938                                                                              0.803                              D       1.936   0.793   1.937  0.798  1.937                                                                              0.806                              E       1.935   0.801   1.936  0.803  1.934                                                                              0.806                              F       1.937   0.798   1.938  0.798  1.937                                                                              0.797                              G       1.938   0.802   1.936  0.803  1.937                                                                              0.804                              H       1.937   0.797   1.935  0.795  1.936                                                                              0.796                              I       1.939   0.797   1.938  0.796  1.936                                                                              0.798                              J       1.936   0.800   1.935  0.802  1.934                                                                              0.803                              K       1.924   0.829   1.925  0.843  1.927                                                                              0.844                              L       1.926   0.817   1.927  0.821  1.925                                                                              0.826                              M       1.923   0.823   1.924  0.826  1.926                                                                              0.820                              N       1.937   0.804   1.936  0.802  1.934                                                                              0.803                              O       1.938   0.800   1.935  0.792  1.937                                                                              0.798                              P       1.936   0.795   1.937  0.792  1.939                                                                              0.792                              ______________________________________                                    

EXAMPLE 5

A steel slab having composition D shown in Table 4 was heated to 1400°C., then hot rolled to produce a hot-rolled steel sheet having athickness of 1.8 mm. Then, the steel sheet was subjected to an annealingprocess at 1000° C. for one minute, followed by a pickling. The steelsheet was then rolled by tandem rolling mill to a thickness of 1.3 mm,after which the sheet was divided into coils R and S.

Coil R was treated in accordance with the present invention, while coilS was, as a comparative example, treated according to conventionalprocesses.

Coil R was heated to 200° C. in a furnace, into which an atmosphere ofN₂ had been introduced, and then warm-rolled at temperature of 180° C.Coil S was heated to 200° C. in a furnace having an ambient atmosphere,followed by rolling at a temperature of 180° C. Then, the two coils wereintermediate-annealed at 1100° C. for one minute, after which thetemperature was rapidly lowered to 350° C. at a rate of 40° C./second.The coils were then gradually cooled at a rate of 1.0° C./second, andthereafter cooled with water. Then, a portion of the surface scales wereremoved, and the coils were cold rolled by a Sendzimir mill with 5passes so that the coils had a final thickness of 0.18 mm. At this time,the quantity of the rolling oil was limited so as to raise thetemperature of the steel after the second stand from 150° C. to 180° C.Then, the coils were wound such that the apparatus for winding coil Rwas surrounded by a box-type structure, and N₂ gas was injected to limitthe concentration of oxygen in the atmosphere from 0.5 vol % to 2 vol %,all while maintaining a constant coiling temperature.

As for the coil S, the coil winding apparatus was surrounded by abox-type structure, but an ambient atmosphere was maintained.

Then, the coil was degreased and subjected to a decarburizing annealingprocess at 850° C. for 2 minutes in an atmosphere consisting of 50 vol %H₂ with the balance substantially consisting of N₂, the dew point of theatmosphere being 50° C. Then, MgO containing TiO₂ by 5 wt % and SrSO₄ by3 wt % was, as an annealing separator, applied, after which the coilswere formed and subjected to a final annealing process.

The final annealing process was performed such that the temperature wasmaintained at 840° C. for 25 hours, and then raised to 1200° C. at arate of 15° C./hour in an atmosphere consisting of 25 vol % N₂ and 75vol % H₂, followed by maintaining the coil at 1200° C. for 5 hours in anatmosphere of H₂.

After the final annealing process had been completed, non-reactedportions of the separator were removed from the two coils, and tensioncoating liquid containing magnesium phosphate and colloidal silica wasapplied. Then, a flattening annealing process, which also baked thecoated material, was performed at 800° C. for one hour.

Results of the magnetic characteristic evaluations of the leadingportion, the central portion and the tailing end of each coil are shownin Table 6.

                  TABLE 6                                                         ______________________________________                                                LEADING  CENTRAL                                                              END      PORTION     TAILING END                                                B.sub.g                                                                              W.sub.17/50                                                                           B.sub.g                                                                            W.sub.17/50                                                                          B.sub.g                                                                            W.sub.17/50                         COILS     (T)    (W/kg)  (T)  (W/kg) (T)  (W/kg)                              ______________________________________                                        EXAMPLE   1.922  0.738   1.924                                                                              0.722  1.923                                                                              0.735                               (COIL R)                                                                      COMPARATIVE                                                                             1.916  0.765   1.846                                                                              0.985  1.918                                                                              0.773                               EXAMPLE                                                                       (COIL S)                                                                      ______________________________________                                    

As shown in Table 6, the coil according to the present invention wasfree from magnetic characteristic deterioration in the central portionof the coil. However, the coil produced as a Comparative exampleexhibited magnetic characteristic deterioration in the central portionthereof.

EXAMPLE 6

A grain-oriented silicon steel sheet slab, consisting of C by 0.075 wt%, Si by 3.35 wt %, Mn by 0.07 wt %, S by 0.003 wt %, P by 0.003 wt %,Al by 0.025 wt %, Se by 0.020 wt %, Sb by 0.025 wt % and N by 0.008 wt %and the balance substantially consisting of Fe, was heated to 1410° C.,then hot rolled to produce a hot-rolled steel sheet having a thicknessof 2.2 mm. The hot-rolled coil was annealed in an atmosphere in whichtown gas was burnt at 1150° C. for 40 seconds, and then mist water wassprayed to rapidly cool the coil to 70° C. at a cooling rate of 30°C./second. Then, the coil was pickled in a water solution of HCl.

Then, the coil was divided into coils a, b, c, d and e, each coil beingrolled with six passes by a Sendzimir mill. The rolls of the mill were80 mm in diameter, and had a temperature of 100° C. to 230° C. The coilshad a final thickness of 0.26 mm.

Divided coil a was wound at the following temperatures: 80° C. for thefirst pass, 124° C. for the second pass, 179° C. for the third pass,216° C. for the fourth pass, 220° C. for the fifth pass, and116° C. forthe sixth pass. Immediately before winding at the second, third andfourth passes. N₂ gas was sprayed across the upper and lower surfaces ofthe steel sheet to remove liquid on the surfaces of the steel sheet by agas-knife effect.

Divided coil b was wound at the following temperatures: 83° C. for thefirst pass, 120° C. for the second pass, 193° C. for the third pass,212° C. for the fourth pass, 218° C. for the fifth pass, and 107° C. forthe sixth pass. Immediately before winding at the fourth, fifth andsixth passes, N₂ gas was sprayed on the upper and lower surfaces of thesteel sheet to remove liquid on the surfaces of the steel sheet by agas-knife effect.

Divided coil c was wound at the following temperatures: 73° C. for thefirst pass, 122° C. for the second pass, 188° C. for the third pass,216° C. for the fourth pass, 212° C. for the fifth pass, and 113° C. forthe sixth pass. Immediately before winding at the fifth and sixthpasses, suction rolls were used to remove liquid on the surfaces of thesteel sheet.

Divided coil d was wound at the following temperatures: 84° C. for thefirst pass, 136° C. for the second pass, 192° C. for the third pass,209° C. for the fourth pass, 216° C. for the fifth pass, and 121° C. forthe sixth pass. Immediately before winding at the sixth pass, suctionrolls were used to remove liquid on the surface of the steel sheet.

Divided coils a, b, c and d are examples of the present invention.

Divided coil e was wound at the following temperatures: 86° C. for thefirst pass, 125° C. for the second pass, 185° C. for the third pass,224° C. for the fourth pass, 208° C. for the fifth pass, and 122° C. forthe sixth pass. No measures for removing liquid from the surfaces of thesteel sheet were undertaken.

Divided coils a, b, c, d and e were all degreased after being rolled,and subjected to a decarburizing annealing process at 840° C. for 2minutes in an atmosphere of 50 vol % H₂ with the balance substantiallyconsisting of N₂, the dew-point of the atmosphere being 48° C. Then, MgOcontaining TiO₂ by 8 wt % was, as an annealing separator, applied, afterwhich the coils were formed and subjected to a final annealing process.

The final annealing process was performed such that the coils weremaintained at 850° C. for 15 hours in an atmosphere of N₂, andthereafter the temperature was raised to 1200° C. at a temperaturerising rate of 15° C./hour in an atmosphere consisting of 15 vol % N₂and 85 vol % H₂. Then, the temperature was maintained at 1200° C. for 5hours in an atmosphere of H₂, after which the temperature was lowered.

Non-reacted portions of the separator were removed, and a tensioncoating material was applied. The steel was then subjected to aflattening process at 800° C. for one minute.

Results of the magnetic characteristic evaluations of the leadingportion, the central portion and the tailing end of each coil are shownin Table 7 and FIG. 4.

As shown in Table 7, the conventional example (coil e) exhibitedmagnetic characteristic deterioration in the central portion thereof,whereas the coil according to the present invention was free from anysuch deterioration.

As can be understood from FIG. 4, an excellent effect was obtained evenif the liquid removal process was performed only on one pass.

                                      TABLE 7                                     __________________________________________________________________________                             MAGNETIC                                             NUMBER OF PASSES         CHARACTERISTICS                                           SUBJECTED LIQUID                                                                         POSITION IN THE                                                                        Bg   W.sub.17/50                                     SAMPLE                                                                             REMOVAL PROCESS                                                                          COIL     (T)  (W/kg)                                                                             REMARKS                                    __________________________________________________________________________    a    4          Leading end                                                                            1.943                                                                              0.908                                                                              Example of this                                            Central portion                                                                        1.942                                                                              0.910                                                                              invention                                                  Tailing end                                                                            1.944                                                                              0.896                                           b    3          Leading end                                                                            1.943                                                                              0.905                                                                              Example of this                                            Central portion                                                                        1.940                                                                              0.911                                                                              invention                                                  Tailing end                                                                            1.944                                                                              0.898                                           c    2          Leading end                                                                            1.943                                                                              0.901                                                                              Example of this                                            Central portion                                                                        1.934                                                                              0.928                                                                              invention                                                  Tailing end                                                                            1.940                                                                              0.912                                           d    1          Leading end                                                                            1.940                                                                              0.912                                                                              Example of this                                            Central protion                                                                        1.918                                                                              1.007                                                                              invention                                                  Tailing end                                                                            1.938                                                                              0.915                                           e    0          Leading end                                                                            1.914                                                                              1.014                                                                              Comparative example                                        Central portion                                                                        1.852                                                                              1.235                                                           Tailing end                                                                            1.920                                                                              0.988                                           __________________________________________________________________________

EXAMPLE 7

Four steel slabs respectively having the compositions A to D shown inTable 4 were heated to 1420° C., then hot rolled to produce hot-rolledsteel sheets each having a thickness of 2.0 mm. The steel sheets werepickled, surface scales removed, and then a first cold rolling processwere performed so that each of the steel sheets had an intermediatethickness of 1.50 mm. Thereafter, an intermediate annealing process wasperformed at 1100° C. for 50 seconds, and mist water was applied, thuslowering the steel temperature to 350° C. at a rate of 40° C./second.Then, the temperature was maintained at 350° C. for 20 seconds, afterwhich the steel sheets were cooled by immersing them in 90° C. hotwater. The steel sheets were then immediately pickled with an acid in a80° C.-water-solution of 15 wt % HCl so that a major portion of thescales were removed.

Then, the steel sheets were rolled with six passes by a Sendzimir millso that each of the steel sheets had a final thickness of 0.22 mm. Atthis time, the quantity of the rolling oil was limited so as to raisethe temperature of the steel after the second pass from 150° C. to 230°C.

Each of the coils was divided into two sections, one of the coils ofeach pair being rolled using conventional rolling oil. On the otherhand, the other coil of each pair was rolled using rolling oil to whichwas added an ester of succinic acid by 2 wt % as an oxidation inhibitorfor the steel sheet.

Each coil was then degreased and subjected to a decarburizing annealingprocess at 850° C. for two minutes in an atmosphere consisting of 60 vol% H₂ with the balance substantially consisting of N₂, the dew-point ofthe atmosphere being 45° C. Then, MgO containing TiO₂ by 5 wt % andSr(OH)₂ •SH₂ O by 3 wt % was applied as an annealing separator, and thencoils were formed and subjected to a final annealing process.

The final annealing process was performed such that the coils weremaintained at 850° C. for 20 hours, then the temperature was raised to1200° C. at a rate of 15° C./hour in an atmosphere consisting of 25 vol% N₂ and 75 vol % Then, the temperature was maintained at 1200° C. for 5hours in an atmosphere of H₂.

After the final annealing process was completed, non-reacted portions ofthe separator were removed, and tension coating liquid containingmagnesium phosphate and colloidal silica was applied. The coils werethen subjected to a flattening annealing process, which also baked thecoating liquid, at 800° C. for one hour.

Results of the magnetic characteristic evaluations of the leadingportion, the central portion and the tailing end of each coil are shownin Table 8.

As is shown in Table 8, the comparative examples exhibited magneticcharacteristic deterioration in the central portion of the coils.Conversely, the examples of the present invention showed no suchdeterioration.

                                      TABLE 8                                     __________________________________________________________________________                        MAGNETIC                                                  ADDITION OF         CHARACTERISTICS                                                OXIDATION                                                                             POSITION IN                                                                          Bg   W.sub.17/50                                          SAMPLE                                                                             INHIBITOR                                                                             THE COIL                                                                             (T)  (W/.sub.kg)                                                                        REMARKS                                         __________________________________________________________________________    A    Not added                                                                             Leading end                                                                          1.918                                                                              0.903                                                                              Comparative example                                          Central portion                                                                      1.867                                                                              0.978                                                                              Comparative example                                          Tailing end                                                                          1.917                                                                              0.905                                                                              Comparative example                                  Added   Leading end                                                                          1.925                                                                              0.847                                                                              Example of this invention                                    Central portion                                                                      1.923                                                                              0.850                                                                              Example of this invention                                    Tailing end                                                                          1.924                                                                              0.848                                                                              Example of this invention                       B    Not added                                                                             Leading end                                                                          1.914                                                                              0.924                                                                              Comparative example                                          Central portion                                                                      1.856                                                                              0.983                                                                              Comparative example                                          Tailing end                                                                          1.912                                                                              0.918                                                                              Comparative example                                  Added   Leading end                                                                          1.925                                                                              0.845                                                                              Example of this invention                                    Central portion                                                                      1.922                                                                              0.856                                                                              Example of this invention                                    Tailing end                                                                          1.923                                                                              0.843                                                                              Example of this invention                       C    Not added                                                                             Leading end                                                                          1.923                                                                              0.857                                                                              Comparative example                                          Central portion                                                                      1.884                                                                              0.948                                                                              Comparative example                                          Tailing end                                                                          1.924                                                                              0.852                                                                              Comparative example                                  Added   Leading end                                                                          1.939                                                                              0.792                                                                              Example of this invention                                    Central portion                                                                      1.937                                                                              0.808                                                                              Example of this invention                                    Tailing end                                                                          1.938                                                                              0.802                                                                              Example of this invention                       D    Not added                                                                             Leading end                                                                          1.923                                                                              0.858                                                                              Comparative example                                          Central portion                                                                      1.887                                                                              0.944                                                                              Comparative example                                          Tailing end                                                                          1.922                                                                              0.856                                                                              Comparative example                                  Added   Leading end                                                                          1.938                                                                              0.797                                                                              Example of this invention                                    Central portion                                                                      1.937                                                                              0.811                                                                              Example of this invention                                    Tailing end                                                                          1.938                                                                              0.798                                                                              Example of this invention                       __________________________________________________________________________

EXAMPLE 8

Steel slabs respectively having composition E to J shown in Table 4 wereheated to 1390° C., followed by hot rolling to produce hot-rolled steelsheets each having a thickness of 2.0 ram. Then, an annealing processwas performed at 1180° C. for 30 seconds, after which the steel sheetswere rapidly cooled with mist water to room temperature at a rate of 40°C./second. Then, the steel sheets were pickled to remove a major portionof the scales.

The foregoing coils were rolled with six passes by a Sendzimir mill to afinal thickness of 0.35 mm. Heat generated due to the rolling operationwas used to perform a warm rolling at 150° C. to 230° C. in the secondand ensuing passes.

A fatty acid of tallow was, by 0.5 wt %, added to the rolling oil andthe roll coolant oil to act as an oxidation inhibitor for the steelsheet.

During the winding by the Sendzimir mill, the coil winding apparatus wassurrounded by a box-type structure into which N₂ gas was injected sothat the concentration of oxygen in the atmosphere was limited to 0.1vol % to 1 vol %.

Each coil was then degreased and subjected to a decarburizing annealingprocess at 850° C. for two minutes in an atmosphere consisting of 50 vol% H₂ with the balance substantially consisting of N₂, the dew-point ofthe atmosphere being 55° C. Then, MgO containing TiO₂ by 8 wt % wasapplied as an annealing separator, followed by winding the coils andsubjecting them to a final annealing process.

The final annealing process was performed such that the coils wereheated to 850° C. at a rate of 30° C./hour in an atmosphere of N₂, afterwhich the temperature was raised to 1200° C. at a rate of 15° C./hour inan atmosphere consisting of 25 vol % N₂ and 75 vol % H₂. The temperaturewas then maintained at 1200° C. for 5 hours in an atmosphere of H₂.

After the final annealing process had been completed, non-reactedportions of the separator were removed, and tension coating liquidcontaining magnesium phosphate and colloidal silica was applied. Aflattening annealing process, which also baked the coating liquid, wasperformed at 800° C. for one minute.

Results of the magnetic characteristic evaluations at the leadingportion, the central portion and the tailing end of each coil are shownin Table 9.

As is shown in Table 9, the magnetic characteristics of all samples werenot deteriorated at the central portion of each coil.

                  TABLE 9                                                         ______________________________________                                        POSITION IN THE COIL                                                                          CENTRAL                                                       LEADING END     PORTION       TAILING END                                             Bg      W.sub.17/50                                                                           Bg     W.sub.17/50                                                                          Bg   W.sub.17/50                        STEEL No.                                                                             (T)     (W/kg)  (T)    (W/kg) (T)  (W/kg)                             ______________________________________                                        E       1.938   1.041   1.939  1.044  1.940                                                                              1.032                              F       1.935   1.073   1.935  1.098  1.936                                                                              1.084                              G       1.937   1.055   1.937  1.060  1.938                                                                              1.060                              H       1.936   1.083   1.936  1.084  1.937                                                                              1.063                              I       1.940   1.035   1.939  1.043  1.941                                                                              1.035                              J       1.941   1.042   1.940  1.037  1.942                                                                              1.038                              ______________________________________                                    

EXAMPLE 9

Six steel slabs respectively having compositions K to P shown in Table 4were heated to 1390° C., followed by hot rolling to produce hot-rolledsteel sheets each having a thickness of 1.8 mm. Then, the steel sheetswere subjected to an annealing process at 1000° C. for one minute,followed by a pickling. The steel sheets were then wound by a tandemrolling mill having four stands so that each steel sheet had a thicknessof 0.90 mm. At this time, the quantity of the coolant oil was limited soas to gradually raise the temperatures of the steel sheets at the outletof the roll bite to 80° C., 110° C., 150° C. and 210° C., respectively.Furthermore, N₂ gas was sprayed at the roll bite outlet of the finalstand so that liquid on the upper and lower surfaces of each steel sheetwas removed.

The temperature of each of the coils was maintained at 200° C. for onehour in a box-type furnace in a atmosphere of N₂, and then the sametandem mill was used so that each coil had a final thickness of 0.29 mm.At this time, the quantity of the strip coolant oil was again limited togradually raise the temperatures of the steel sheets at the outlet ofthe roll bite to 120° C., 170° C., 210° C. and 220° C., respectively.Then, N₂ gas was sprayed at the roll bite outlet of the final stand sothat liquid on the upper and lower surfaces of the steel sheets wasremoved.

After the cold rolling process had been completed, each coil wasdegreased and subjected a decarburizing annealing process at 850° C. for2 minutes in a furnace, the atmosphere of which consisted of 50 vol % H₂with the balance substantially consisting of N₂, the dew point of whichwas 55° C. Then, MgO, containing TiO₂ by 8 wt % and Sr(OH)₂ •8H₂ O by 3wt %, was applied as an annealing separator, followed by winding thecoils. Then, the coils were subjected to a final annealing process.

The final annealing process was performed such that the coils wereheated to 850° C. at a rate of 30° C./hour in an atmosphere of N₂, andthen the temperature was raised to 1200° C. at a rate of 15° C./hour inan atmosphere consisting of 25 vol % N₂ and 75 vol % H₂. Then, thetemperature was maintained at 1200° C. for 5 hours in an atmosphere ofH₂.

After the final annealing process had been completed, non-reactedportions of the separator were removed, and tension coating liquidcontaining aluminum phosphate and colloidal silica was applied. Thecoils were then subjected to a flattening annealing process at 800° C.for one minute, which also baked the coating liquid.

Results of the magnetic characteristic evaluations of the leadingportion, the central portion and the tailing end of each coil are shownin Table 10.

As is shown in Table 10, the magnetic characteristics of all sampleswere not deteriorated at the central portion of each coil.

                  TABLE 10                                                        ______________________________________                                        POSITION IN THE COIL                                                                          CENTRAL                                                       LEADING END     PORTION       TAILING END                                             Bg      W.sub.17/50                                                                           Bg     W.sub.17/50                                                                          Bg   W.sub.17/50                        STEEL No.                                                                             (T)     (W/kg)  (T)    (W/kg) (T)  (W/kg)                             ______________________________________                                        K       1.942   0.947   1.942  0.948  1.943                                                                              0.945                              L       1.932   0.972   1.932  0.974  1.931                                                                              0.975                              M       1.930   0.980   1.929  0.980  1.931                                                                              0.976                              N       1.952   0.941   1.951  0.940  1.953                                                                              0.940                              O       1.941   0.952   1.940  0.959  1.940                                                                              0.963                              P       1.942   0.948   1.941  0.948  1.941                                                                              0.956                              ______________________________________                                    

According to the present invention, when a grain-oriented silicon steelsheet containing Al is manufactured in such a manner that a heat effecttreatment is performed in a cold rolling process, deterioration in themagnetic characteristics occurring at the central portion of the coilcan effectively be prevented. Thus, a grain-oriented silicon steel sheethaving excellent magnetic characteristics for the overall length of thecoil can be obtained.

Although this invention has been described in connection with specificforms thereof, equivalent steps may be substituted, the sequence of thesteps may be varied, and certain steps may be used independently ofothers. Further, various other control steps may be included, allwithout departing from the spirit and scope of the invention defined inthe appended claims.

What is claimed is:
 1. A method of manufacturing a grain-orientedsilicon steel sheet which exhibits excellent magnetic characteristicsover the entire length of a coil thereof, comprising the steps of:hotrolling a silicon steel slab that contains aluminum to form a steelsheet; annealing said steel sheet, as the need arises; cold rolling saidsteel sheet to a final thickness, said cold rolling comprising one passor plural passes including an intermediate annealing; performing a heateffect treatment, said heat effect treatment being selected from thegroup consisting of a coil heating process performed before said coldrolling, a warm rolling process performed during said cold rolling, anaging heat treatment performed during said cold rolling, a heatmaintenance process performed during said cold rolling, and a heatmaintenance process performed after said cold rolling, said heat effecttreatment being performed in an atmosphere having an oxygenconcentration of about 10 vol % or lower; performing a decarburizingannealing on said steel sheet after said cold rolling and said heateffect treatment; and performing a final annealing after saiddecarburizing annealing on said steel sheet; whereby nitriding of saidsteel sheet during said final annealing is minimized so that excellentmagnetic characteristics are maintained over the entire length of saidsteel sheet.
 2. A method of manufacturing a grain-oriented silicon steelsheet which exhibits excellent magnetic characteristics over the entirelength of a coil thereof, comprising the steps of:hot rolling a siliconsteel slab that contains aluminum to form a steel sheet; annealing saidsteel sheet, as the need arises; cold rolling said steel sheet to afinal thickness, said cold rolling comprising one stage or plural stagesincluding an intermediate annealing, said steel sheet having a liquidthereon during said cold rolling, said cold rolling also including astep wherein the amount of said liquid is reduced during at least onepass of said cold rolling, said step being performed in a downstreamregion from a roll bite outlet of said cold rolling to a position atwhich said steel is wound; performing a heat effect treatment, said heateffect treatment being selected from the group consisting of a coilheating process performed before said cold rolling, a warm rollingprocess performed during said cold rolling, an aging heat treatmentperformed during said cold rolling, a heat maintenance process performedduring said cold rolling, and a heat maintenance process performed aftersaid cold rolling; performing a decarburizing annealing on said steelsheet; and performing a final annealing on said steel sheet; wherebynitriding of said steel sheet during said final annealing is minimizedso that excellent magnetic characteristics are maintained over theentire length of said steel sheet.
 3. A method of manufacturing agrain-oriented silicon steel sheet which exhibits excellent magneticcharacteristics over the entire length of a coil thereof, comprising thesteins of:hot rolling a silicon steel slab that contains aluminum toform a steel sheet; annealing said steel sheet, as the need arises; coldrolling said steel sheet to a final thickness, said cold rollingcomprising one stage or plural stages including an intermediateannealing, wherein at least one of the group consisting of rolling oil,roll coolant oil and strip coolant oil is used in said cold rolling, andan inhibitor for inhibiting oxidation of said steel sheet is added to atleast one of said group; performing a heat effect treatment, said heateffect treatment being selected from the group consisting of a coilheating process performed before said cold rolling, a warm rollingprocess performed during said cold rolling, an aging heat treatmentperformed during said cold rolling, a heat maintenance process performedduring said cold rolling, and a heat maintenance process performed aftersaid cold rolling; performing a decarburizing annealing on said steelsheet; and performing a final annealing on said steel sheet; wherebynitriding of said steel sheet during said final annealing is minimizedso that excellent magnetic characteristics are maintained over theentire length of said steel sheet.